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TL;DR
Osaka City confirmed on November 13 that 1,078,713 email addresses associated with its administrative online system were exposed in a breach at Arara Message, an email service used by system provider TKC. The city and TKC say the exposure involved email addresses only; they say message contents, My Number data and application information were not leaked.
Osaka City confirmed November 13 that 1,078,713 email addresses linked to its administrative online system were exposed after an attacker accessed Arara Message, an email delivery service used by system provider TKC Corporation. The city and TKC say the leak involved email addresses only, not message contents, My Number identifiers or application data.
The city’s report counts addresses that received emails from the system between July 1 and October 31, 2026. Those messages covered registration, changes to email addresses or passwords, logins, applications, application-status updates and announcements. Osaka said the total is high partly because it sent a notice to all registered users on September 16. The city’s explanation means people who held accounts then should treat their addresses as potentially included; it has not published a way for individuals to check their status.
The breach occurred at Arara Message, which TKC used to deliver system emails, rather than in Osaka’s own online system, according to the city and the companies. Arara said 81,329,467 email addresses were taken across its service after duplicates were removed, involving as many as about 380 client companies. That is a count of addresses, not people. The number of unique individuals represented has not been disclosed.
Osaka said it identified its affected addresses using information Arara provided to TKC, which sorted and matched the data to systems and customers. TKC said it received the leaked addresses from Arara on November 4 and began classifying them that day. Osaka says its own system has not been found to have been accessed, and TKC says no intrusion into its systems was found.
Osaka email leak · Oct 22 – Nov 13, 2026
How Osaka’s user emails leaked through a vendor’s vendor
No break-in at the city’s system or at its provider has been found, they say. The attack hit the email service one step further down.
Where the addresses were (schematic)
What happened
- Oct 22Arara Message briefly stopped for a PHP vulnerability responseper TKC
- Oct 28Arara confirms unauthorized access; its service starts sending fake card emails
- Oct 31Arara shuts its whole service down; from about 17:40 Osaka’s system cannot send email
- Nov 2TKC tells the city that user addresses may have leaked
- Nov 4Only email addresses affected, says TKC; Arara hands it the leaked addresses
- Nov 13The city confirms the count and warns of fake emails in its name
info-osaka-online@city.osaka.lg.jp between July 1 and Oct 31, 2026. Addresses, not people.Sources: City of Osaka, notice and press releases of Nov 3, 4 and 13, 2026; TKC Corporation, reports one to six; Arara and PayCloud Holdings disclosures. The chain is a schematic.
Graphic: Powered by Thorsten Meyer AI
What the Exposed Addresses Mean
An email address alone does not provide access to an Osaka account, and the city says passwords and application data were not exposed in this incident. But the addresses can be used to target users with messages that appear connected to city services. People who received system emails during the stated period may therefore face a greater risk of convincing impersonation or phishing attempts, even though the source material does not establish that such messages have targeted Osaka users.
Users should treat unexpected messages claiming to be from the city with caution, particularly if they ask for passwords, payment-card information or other personal details. Avoid using links in suspicious emails to sign in or provide information. Instead, reach the city’s services through its official website or a known address. The city has said it cannot answer requests to confirm whether a particular email address was affected.
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How the Vendor Breach Reached Osaka
The incident involved several organizations: Osaka operated the online service, TKC provided the system, and Arara Message handled email delivery. The city says the unauthorized access occurred at that third-party service. This distinction matters: the confirmed exposure does not, on the information released, show that attackers entered Osaka’s system or TKC’s own environment.
TKC’s account of the sequence says Arara reported unauthorized access on October 28. Arara later found that data had also been taken through its service and shut down the environment used for local-government systems on October 31. Osaka’s system could no longer send email from about 5:40 p.m. that day, according to the timeline in the source material. Arara subsequently revised its estimate of the total number of exposed addresses as its investigation progressed.
Osaka’s first notices came on November 3 and 4, while the city’s final report confirming its own total was published on November 13. The report follows earlier, broader estimates from TKC and Arara; the final Osaka figure is specific to addresses associated with the city’s system, not the entire vendor incident.
“Users’ email addresses, 1,078,713 records.”
— Osaka City, in its November 13 report
What the Public Record Does Not Show
The city has confirmed the number of addresses it matched to the incident, but it has not disclosed how many distinct people those addresses represent. The total across Arara’s service also refers to email addresses rather than individuals; some people may have more than one address, and the source material provides no person-level count.
Osaka says it is considering individual notices to affected users and has requested a detailed report from TKC on the cause, scope and measures to prevent a repeat. The material provided does not say whether individual notices have since been sent, disclose technical details about the attacker or establish that any Osaka user suffered follow-on fraud. The city also says it cannot confirm whether a particular address was included.
The City’s Vendor Review
Osaka says it will review how personal information is handled by external services, vendor safety measures and coordination with contractors. It has asked TKC for a fuller account of the incident and the steps taken to prevent a recurrence. The source material does not give a deadline for that report or for the city’s review.
For users, the immediate step is to be alert to unexpected messages claiming to come from Osaka City. Do not enter login credentials or card details through a link in an unsolicited email; verify requests through the city’s official website. Further information may follow if Osaka publishes the vendor report, changes its notification plans or updates its guidance.
Key Questions
Was my email address exposed?
If you received an email from Osaka’s administrative online system between July 1 and October 31, 2026, your address may be included. The city says it cannot confirm an individual address through inquiries.
What information does Osaka say was leaked?
Osaka and TKC say the confirmed exposure was limited to email addresses. They say email contents, My Number identifiers and application data were not leaked.
Was Osaka City’s online system hacked?
The city says no break-in at its online system has been found. It attributes the exposure to unauthorized access at Arara Message, the email delivery service used by TKC.
How many people were affected?
Osaka confirmed 1,078,713 email addresses, not a count of people. The city has not published the number of unique individuals represented by those addresses.
What should I do about an email claiming to be from Osaka?
Treat unexpected messages cautiously. Do not follow suspicious links or provide passwords or payment-card details; check information through Osaka City’s official website instead.
Source: ThorstenMeyerAI.com
